SAN FRANCISCO – Federal prosecutors have launched a crackdown on pot dispensaries in California, warning the stores that they must shut down in 45 days or face criminal charges and confiscation of their property even if they are operating legally under the state’s 15-year-old medical marijuana law.

In an escalation of the ongoing conflict between the U.S. government and the nation’s burgeoning medical marijuana industry, California’s s four U.S. attorneys sent letters Wednesday and Thursday notifying at least 16 pot shops or their landlords that they are violating federal drug laws, even though medical marijuana is legal in California. The attorneys are scheduled to announce their coordinated crackdown at a Friday news conference. None of the dispensaries mentioned in the letters are in Orange County.

Their offices refused to confirm the closure orders. The Associated Press obtained copies of the letters that a prosecutor sent to 12 San Diego dispensaries. They state that federal law “takes precedence over state law and applies regardless of the particular uses for which a dispensary is selling and distributing marijuana.”

Under United States law, a dispensary’s operations involving sales and distribution of marijuana are illegal and subject to criminal prosecution and civil enforcement actions,” letters signed by U.S. Attorney Laura Duffy in San Diego read. “Real and personal property involved in such operations are subject to seizure by and forfeiture to the United States … regardless of the purported purpose of the dispensary.”

Damian Nassiri, an attorney representing several medical marijuana collectives in Lake Forest, said the crackdown is not surprising.

While Nassiri hasn’t heard that the federal prosecutors are targeting any Orange or Los Angeles county dispensaries yet, it may be sign of things to come, he said.

“San Diego has a real aggressive stance against medical marijuana dispensaries,” he said. “It seems like they’re going district by district with the DEA agents. They also seem to be going after the big fish — the dispensaries that are arguably not in compliance with state law and who arguably are operating for profit.”

Nassiri said he believes the action will lead to even more litigation and confusion over what people can or cannot do.

Last week the City Council in Rancho Santa Margarita enacted a 45-day moratorium on any medical marijuana dispensaries attempting to open in the city.

“This is precisely why we’ve asked the council to consider this,” City Manager Steve Hayman said about the federal crackdown. “We want to study all aspects of the law to consider should it be regulated or should it be banned. This is the type of information we’re looking to accumulate.”

The move by federal prosecutors comes a little more than two months after the Obama administration toughened its stand on medical marijuana following a two-year period during which federal officials had indicated they would not move aggressively against dispensaries in compliance with laws in the 16 states where pot is legal for people with doctors’ recommendations.

“This is a big failure by the Obama administration to not really have helped the medical marijuana community,” Nassiri said.

The Department of Justice issued a policy memo to federal prosecutors in late June stating that marijuana dispensaries and licensed growers in states with medical marijuana laws could face prosecution for violating federal drug and money-laundering laws. The effort to shutter California dispensaries appears to be the most far-reaching effort so far to put that guidance into action.

“This really shouldn’t come as a surprise to anyone. The Administration is simply making good on multiple threats issued since President Obama took office,” said Kevin Sabet, a former adviser to the president’s drug czar who is a fellow at the University of Pennsylvania’s Center for Substance Abuse Solutions. “The challenge is to balance the scarcity of law enforcement resources and the sanctity of this country’s medication approval process. It seems like the Administration is simply making good on multiple statements made previously to appropriately strike that balance.”

Greg Anton, a lawyer who represents a Marin Alliance for Medical Marijuana, said the 14-year-old dispensary’s landlord received an “extremely threatening” letter Wednesday invoking a federal law that imposes additional penalties for selling drugs within 1,000 feet of schools, parks and playgrounds.

The landlord was ordered to evict the pot club or risk imprisonment, plus forfeiture of the property and all the rent he has collected while the dispensary has been in business, Anton said.

The Marin Alliance’s founder “has been paying state and federal taxes for 14 years, and they have cashed all the checks,” he said. “All I hear from Obama is whining about his budget, but he has money to do this which will actually reduce revenues.”